On plasma screens, the grey level is not produced in a conventional manner using amplitude modulation of the signal but rather temporal modulation of this signal, by exciting the corresponding pixel for a greater or lesser time depending on the level desired. It is the phenomenon of integration by the eye which makes it possible to render this grey level. This integration is performed during the frame scan time.
The eye actually integrates much faster than the frame duration and is therefore liable to perceive, in cases of particular transition of the addressing bits, variations in level which do not reflect reality. Contour defects or "contouring" as it is known, may thus appear in the moving images. These defects may be compared to poor temporal restitution of the grey level. More generally, false colours appear on the contours of objects, each of the cells of a colour component possibly being subject to this phenomenon. This phenomenon is even more harmful when it occurs in relatively homogeneous zones.
A simple theoretical solution for limiting these problems of the appearance of false contours is to multiply the number of sub-scans so that the disturbances related to the modifications of the video level from one frame to another are made minimal. Such a solution has formed the subject of a patent application in France filed by the Applicant on Apr. 25, 1997 under national registration number 97 05166. By virtue of the simultaneous addressing of two consecutive lines in respect of bits of the column addressing word and by virtue of the sub-scans thus saved, allowing transcoding of the column control words over a greater number of bits, it is possible to reduce the weights of the most significant bits.
The losses of resolution which are caused by this may be limited by using the redundancy possibilities of the codes for the recoding of the grey level. However, it is not possible to curb the magnitude of these losses of resolution.
The purpose of the invention is to alleviate the aforesaid drawbacks.